1. Field of the Invention
The present invention relates to the providing of text-based messages originating from a network entity to network terminals using speech encoding.
2. Description of the Prior Art
Due to digital cellular phone systems being optimized for speech signals and radio transmissions thereof may be subject to relative high error rates, the known transmission of text telephone originated messages using known speech transmission techniques in cellular networks is, in some cases, unsatisfactory. The received messages have an expectedly high character error rate (CER). One reason for this is that the digital cellular phones use speech coding in order to compress the signal. Since the coding is optimized for speech-like signals, text messages using the speech path of a cellular network are subject to distortion. A further problem is transmission errors and the applied error correction in digital cellular systems in poor channel conditions. The repetition of speech frames results in character errors and error propagation. Traditional text telephone modulation technique, designed years ago for the public switched telephone network (PSTN) lines, was not developed for sophisticated speech-optimized transmission channels.
For various reasons, text telephone transmission methods using the speech path are desired in view of text telephony being acknowledged as a way to make contact for emergency services. Emergency services in wireless telephones are so far only defined for speech calls. Alternating speech and text in a call is desired. A simple way to accomplish this goal without special service support (like multimedia) is alternating use of the speech channel for speech and text.
FIG. 1 illustrates FIG. 1 of the 3GPP Technical Specification TS 26.226 v5.0.0 (2001-03) which is incorporated herein by reference in its entirety. An overall architecture is illustrated for supporting text telephone and speech communications via cellular telephone.
The cellular text telephone (CTM) adaptor 32, including a transmitter and receiver, supports text and voice communications using speech encoding in cellular telephone systems. The CTM adaptor may be used with the practice of the present invention in a preferred embodiment thereof as described below.
The system of FIG. 1 includes a mobile subscriber terminal 10 which includes both an input 12 and an output 14 for speech which are respectively utilized for transmitting and receiving speech. Text telephone 16 provides text which is selectively transmitted after the closing of switch 18. Element 20 includes a text telephone detector 22 which receives an input of either text from the text telephone 16 or speech from the speech input 14. If text is detected by the text telephone detector, switch S3 is opened, causing the output of text by the text telephone detector. On the other hand, if the text telephone detector 22 does not detect text, switch S3 is closed by the text telephone detector which bypasses the text telephone detector to prevent outputting of text. A CTM transmitter 24 processes the input of text into an output CTM signal which is encoded with coding chosen to not introduce unacceptably high CER during subsequent encoding by the speech encoder 26. The combination of the CTM transmitter 24 and speech encoder 26 provide for a robust transmission of textual information over the speech channel 28 without the problems that characterize the prior art. If speech input 12 is providing sound-based information, switch S1 is closed by the CTM transmitter 24 so that speech totally bypasses the text telephone detector 22 and the CTM transmitter 24 and is added to the output of the CTM transmitter 24 and input to the speech encoder 26 by adder 30. It is therefore seen that the CTM adapter 32 when transmitting either speech or text modulates the output with a traditional speech encoder 26.
The transmitted speech encoded signal is transmitted over an air interface using encoding produced, for example, by an AMR speech coder, to a network part 34 which may be a media gateway or transcoding equipment. A speech decoder processes the encoded transmission on the speech channel and provides a PCM encoded input thereof to the CTM receiver 38 of a CTM adapter 32. The CTM receiver 38 detects whether or not text is transmitted. When text is detected by the CTM receiver 38, switch S2 is opened causing the output of text from the CTM receiver 38 to be input to a text telephone regenerator 42 which regenerates the input of text from the CTM receiver 38 into a form reproducible by text telephone 44. Text telephone 44 may be part of any network 46, including the mobile network in which text telephone 16 is resident providing text telephone support thereof. If the output from the speech decoder 36 is not detected by the CTM receiver 38 as being text, switch S2 is closed by the CTM receiver 38 and similarly, switch S4 is closed by the text telephone regenerator 42 resulting in speech being input by adder 48 into the network 46. Assuming that text is transmitted, the text is reproduced by text telephone 44 in the well-known manner. If speech is present, speech is outputted to the user by a speaker or other element (not illustrated).
The operation of the system of FIG. 1 for the reception of speech or text originated by the telephone connected to the network 46 is the reverse of the above process. The same elements are used to encode and decode the transmission of text or speech in speech encoded form in a reverse direction but in the same manner as described above during the delivery of text or speech to the mobile terminal 10.
The architecture of the CTM transmitter 24 and the CTM receiver 38 is described in the aforementioned TS 26.226 beginning in Section 8 thereof and therefore is not described in detail herein.
CTM works well between telephone users. However, the CTM standard does not describe a mechanism by which messages which are conventionally transmitted by telephone networks in a non-text form, such as voice, tone, or sound, are alternatively transmitted as text messages to users of text telephones. Therefore, while the prior art has a mechanism for transmitting textual messages to telephone users, there is no current mechanism which permits a message which is sent traditionally by a network in non-textual form to users of ordinary telephones to be selectively encodable as a textual message which is speech encoded and transmitted over a speech channel to users of text telephones so as to enable those users to be notified of the content of the traditional sound-based messages originating from a telephone network via textual format.